It is further known that the production batch related error of a disposable blood pump segment tube can be reduced by “calibrating” the pump delivery with every tube segment before use. However, this requires an accurate measurement system. Usually, in acute renal replacement therapy apparatus the patient balance is controlled by using a weight measurement system. This system is also used for automatic priming and testing the substitution/dialysis fluid circuit in preparation, but is not applied for the blood delivery system. For priming the blood side tubing usually a filled saline bag is applied to the IV pole connected to the arterial line of the blood line. A waste bag is connected to the venous line and is placed on a load cell to determine the weight of the fluid contained in the waste bag and to perform the “calibration” of the real peristaltic blood pump delivery. It is a disadvantage that a very slow automatic calibration and priming operation is the result and that it may require manual intervention, since the calibration can only be started after the disposable system is primed, i.e. when the fluid reaches the waste bag. Such a calibration procedure therefore requires extra priming time. Additionally, the accuracy of measurement is not satisfying. This is because during priming the system (especially when priming a hemofilter containing small capillary) air removal from the piping is slow. Further, even during the calibration phase air bubbles are replaced by a certain flow of fluid, which certain flow of fluid cannot be determined by the load cell.
Another large effect and disadvantage on the accuracy of the blood flow is the influence of the pump inlet pressure to the behaviour of the piping. A negative pressure may result in some collapsing of the pump segment tube thus making it oval. Such a deformation of the pump segment tube may reduce the cross section area of the latter, which results in a decrease of the delivery volume. To the contrary, positive inlet pressure may result in enlargement of the cross section area, which results in an increase of the delivery volume. Since a flow lower than a desired flow is considered as negative for treatment and the blood pump inlet pressure (“arterial” pressure) usually is negative because of a thin patient connection provided e.g. by a catheter or a needle, especially errors due to a negative inlet pressure should be reduced. This is especially true in acute renal replacement therapies where usually a catheter is connected into a patient's vein.
It is a disadvantage of the current state of the art solutions, that any error of an extracorporeal blood delivery flow due to deviations of the pumping system caused for example by deviations of different production batches usually cannot be compensated before therapy or at least with a time consuming and inaccurate method.